This invention relates generally to power distribution and management for electronic devices and more particularly to an integrated switch and fuse that allows for adaptive selection of a power source from among multiple power sources.
Several standards are extant for system level design of electronic devices, such as personal computers, which are intended to have multiple peripheral devices attached. In some system designs, the computer supplies power not only to its own components but also to peripherals that may be attached to its various expansion slots and busses. In other systems, peripherals are attached that supply their own power. In still other systems, the computer might supply power to itself and to certain peripheral, while other peripherals supply their own power.
Two well known standards for personal computer expansion busses, also known as peripheral busses, are the Universal Serial Bus (USB) and the IEEE Standard 1394. These standards are intended to be flexible to allow for any of the system power designs discussed above. Care must be taken in the design of such flexible systems to assure that a power supply is not overloaded because of excessive power consumption by the various electronic components (such as the computer itself and the peripherals), or that a conflict does not arise between two power supplies attempting to both supply power competitively, or that potentially damaging reverse current flow does not result from a large peripheral power supply driving the computer itself or over-driving other components attached to the bus.
FIGS. 1a and 1b illustrate a typical prior art computer system 10 containing an expansion board 2 which provides for a flexible approach to power distribution and management. The system 10 includes a power supply 4 which provides power to mother board 6. System 10 may be an IBM-compatible personal computer, such as is well known in the art. Many components of system 10, including I/O devices such as keyboard and display, logic and control circuitry, microprocessor, graphics boards, cooling fans, and the like are not essential to an understanding of the present invention and hence are not illustrated for clarity. Mother board 6 receives power from power supply 4, which can take the form of a battery, or a regulated ac power supply, as is well known in the art. Expansion board 2 also receives power from power supply 4 by way of mother board 6 and power bus 8. Alternatively, expansion board 2 could receive power directly from power supply 4 by way of its own dedicated power bus.
As illustrated, expansion board 2 contains three slots, 12, 14, and 16, respectively, each of which provides an electrical interface for a peripheral device, such as peripherals 18, 20 and 22. As shown, each peripheral 18, 20, and 22 is coupled to power bus 8 by way of its respective slot 12, 14, and 16. In this way, power for the peripherals can be supplied by power supply 4, typically under the control and regulation of circuitry contained on mother board 6 and/or expansion board 2.
Note that peripheral 18 contains its own power supply 24 and that peripheral 20 contains its own power supply 26, whereas peripheral 22 does not contain its own power supply. As discussed above, a flexible system design standard will allow for any other of the three peripherals 18, 20, 22 to be supplied either by its own power supply (e.g., 24 and 26), or by the system power supply (i.e. power supply 4), or by the power supply of one of the other power supplies connected to the expansion board. For instance, peripheral 22 could be supplied from system power supply 4 or, if peripheral 20 has a power supply 26 that is at a higher voltage level than that provided by power supply 4, then power supply 26 could provide the power to peripheral 22 as well as to peripheral 20.
FIG. 1b illustrates further details for a typical prior art circuit interface slot 14. Slots 12 and 16 would also be similarly configured. Slot 14 includes a diode 30 and a fuse 32. Diode 30 is a unidirectional current flow device, meaning the device allows current to flow from the computer system to the peripheral (such as peripheral 20 of FIG. 1a) attached to slot 14, but does not allow reverse current to flow from the peripheral back to the computer system. Fuse 32 protects power supply 4 from an over-drive situation that could occur if, for instance, if the slot was accidentally grounded.
Several shortcomings exist with the prior art. For instance, diode 30 introduces a voltage drop across the circuit, thus lessening the power available to the attached peripheral. Likewise, fuse 32 introduces its own IR voltage drop across it. Also, as is known in the art, fuse 32 is typically a re-settable thermal fuse. Such thermal fuses have a very low on-state impedance and have a higher off-state impedance. This off-state impedance is not an open, however, and hence current flow and heat generation will still occur, even during the fuses thermal shut-down state. Also, such fuses tend to have varying characteristics from fuse to fuse (such as off temperature, off-state impedance, and the like), making circuit design difficult. Another shortcoming of prior art power management circuits is the large foot print occupied by the diode and fuse, taking up valuable board space.
Therefore, a need exists in the prior art for a power management circuit that overcomes the above discussed shortcomings in a small, inexpensive solution.
In one aspect, the present invention provides a power distribution circuit comprising a unidirectional switch having an input coupled to a first power supply and an output coupled to a first node of a bi-directional switch. The bi-directional switch has its second node coupled to a second power supply. The circuit also includes first control logic coupled to said uni-directional switch and allowing said uni-directional switch to conduct when said uni-directional switch is forward biased and preventing said uni-directional switch from conducting when said uni-directional switch is reverse biased. The circuit further includes second control logic coupled to said at least one bi-directional switch, said second control logic switching said at least one bi-directional switch to a non-conducting state when an over-current condition is detected.
In another aspect, the invention provides a computer system comprising at least one input/output device, a system power supply, a system power bus, and a mother board coupled to the power supply by said system power bus and receiving power therefrom. The computer system also includes an expansion board coupled to the power supply and to the mother board, a first peripheral connected to the expansion board, wherein the first peripheral comprises a peripheral power supply and peripheral circuitry and a power distribution circuit having an input coupled to said system power supply and an first output coupled to said peripheral power supply. The power distribution circuit comprises an active diode coupled between the input and a first re-settable fuse circuit, the first re-settable fuse being coupled to the first output.